Thermal insulating device

ABSTRACT

A thermal insulating device, and process for making it, are presented. The inventive thermal insulating device is formed of a cylindrically shaped shell comprising multiple layers of a continuous spiral wound anisotropic flexible graphite sheet, the layers of spiral wound graphite sheet being separated by and bonded to a cured resin.

FIELD OF THE INVENTION

[0001] The present invention relates to a thermal insulating device.More particularly, the present invention relates to a thermal insulatingdevice useful for a high temperature reactor, such as a reactor thatutilizes highly reactive chemical gases, such as inorganic halides,especially chlorine and fluorine, in a non-oxidizing atmosphere. Theinventive thermal insulating device includes a shell comprising resinbonded spiral wound continuous flexible graphite sheet.

BACKGROUND OF THE INVENTION

[0002] Graphites are made up of layer planes of hexagonal arrays ornetworks of carbon atoms. These layer planes of hexagonally arrangedcarbon atoms are substantially flat and are oriented or ordered so as tobe substantially parallel and equidistant to one another. Thesubstantially flat, parallel equidistant sheets or layers of carbonatoms, usually referred to as basal planes, are linked or bondedtogether and groups thereof are arranged in crystallites. Highly orderedgraphites consist of crystallites of considerable size: the crystallitesbeing highly aligned or oriented with respect to each other and havingwell ordered carbon layers. In other words, highly ordered graphiteshave a high degree of preferred crystallite orientation.

[0003] Graphites possess anisotropic structures and thus exhibit orpossess many properties that are highly oriented, i.e. directional.Graphites may be characterized as laminated structures of carbon, thatis, structures consisting of superposed layers or laminae of carbonatoms joined together by weak van der Waals forces. In considering thegraphite structure, two axes or directions are usually noted, i.e. the“c” axis or direction and the “a” axes or directions. For simplicity,the “c” axis or direction may be considered as the directionperpendicular to the carbon layers. The “a” axes or directions may beconsidered as the directions parallel to the carbon layers or thedirections perpendicular to the “c” direction. Natural graphites possessa high degree of orientation and hence anisotropy with respect tothermal and electrical conductivity and other properties.

[0004] As noted above, the bonding forces holding the parallel layers ofcarbon atoms together are only weak van der Waals forces. Graphites canbe treated so that the spacing between the superposed carbon layers orlaminae can be appreciably opened up so as to provide a marked expansionin the direction perpendicular to the layers, that is, in the “c”direction and thus form an expanded or intumesced graphite structure inwhich the laminar character is substantially retained.

[0005] Graphite flake which has been greatly expanded and moreparticularly expanded so as to have a final thickness or “c” directiondimension which is up to about 80 or more times the original “c”direction dimension can be formed without the use of a binder intocohesive or integrated sheets, e.g. webs, papers, strips, tapes, or thelike. The formation of graphite particles which have been expanded tohave a final thickness or “c” dimension which is up to about 80 or moretimes the original “c” direction dimension into integrated sheetswithout the use of any binding material is believed to be possible dueto the excellent mechanical interlocking, or cohesion which is achievedbetween the voluminously expanded graphite particles.

[0006] In addition to flexibility, the sheet material, as noted above,has also been found to possess a high degree of thermal anisotropy.Sheet material can be produced which has excellent flexibility, goodstrength and is highly resistant to chemical attack and has a highdegree of orientation.

[0007] Briefly, the process of producing flexible, binderless graphitesheet material comprises compressing or compacting under a predeterminedload and in the absence of a binder, expanded graphite particles whichhave a “c” direction dimension which is up to about 80 or more timesthat of the original particles so as to form a substantially flat,flexible, integrated graphite sheet. Once compressed, the expandedgraphite particles, which generally are worm-like or vermiform inappearance, will maintain the compression set. The density and thicknessof the sheet material can be varied by controlling the degree ofcompression. The density of the sheet material can be within the rangeof from about 0.08 g/cm³ to about 2.0 g/cm³. The flexible graphite sheetmaterial exhibits an appreciable degree of anisotropy, with the degreeof anisotropy increasing upon roll pressing of the sheet material toincreased density. In roll pressed anisotropic sheet material, thethickness, i.e. the direction perpendicular to the sheet surfacecomprises the “c” direction and the directions ranging along the lengthand width, i.e. along or parallel to the surfaces comprises the “a”directions.

SUMMARY OF THE INVENTION

[0008] The present invention comprises a shell, preferably aself-supporting, cylindrically shaped shell, having two ends (denotedfor the sake of convenience as “top” and “bottom”0 useful, for instance,for surrounding a high temperature radiant heat source, such as areactor in which highly chemically active gases are contained. The shellcan be used as a heat shield to reflect radiant heat energy back to thereactor and to minimize loss of thermal energy due to conduction. Theaforementioned shell comprises multiple layers formed from a continuousspiral wound sheet of anisotropic flexible graphite, bonded with a curedresin. The resin is coated on both sides of a thin sheet of heatdecomposable carbon based material that during the fabrication processis co-extensive with the spiral wound sheet of flexible graphite and iscured in situ. The thin sheet of heat decomposable carbon based materialprovides a path for the escape of gases which develop in the course ofin situ curing of the resin; this path, resulting from the aforesaiddecomposition, is provided between the layers of the sheet ofcarbon-based material and spiral wound flexible graphite and furtherenables contact between resin applied on both, i.e. the opposite, sidesof the sheet of heat decomposable carbon-based material in the course ofin situ curing of the resin. This results in a strong continuous bondinglayer of resin between, and co-extensive with, the spiral wound sheet offlexible graphite.

[0009] In a further embodiment of the present invention, a second shellessentially identical to the first shell, except for being larger incross-section, is provided. The second shell is positioned to surroundthe first shell so as to define an annular chamber therebetween, theannular chamber also having two ends (also conveniently denoted “top”and “bottom”) therebetween. Uncompressed particles of expanded graphitecan be provided in the annular chamber as an insulating material,preferably so as to essentially fill the annular chamber. Otherinsulating materials that can be employed include carbon felt, graphitefelt, rigid insulation, ceramic wool fibers, and even gases like argonor air. Indeed, insulation can also be provided by drawing a vacuum inthe annular chamber.

[0010] The annular chamber can be closed, at one or both of its ends, byone or more flexible sheets of graphite that are advantageouslyresin-bonded to one or both of the first and second shells. Preferably,the covering flexible sheets of graphite are themselves multi-layered,with the individual layers bonded to resin, in the same manner as theshell is formed. As was the case with the inventive shell, the resin iscoated on both sides of a thin sheet of heat decomposable carbon basedmaterial that during the fabrication process is co-extensive with thesheet of flexible graphite and is cured in situ. The thin sheet of heatdecomposable carbon based material provides a path for the escape ofgases which develop in the course of in situ curing of the resin; thispath, resulting from the aforesaid decomposition, is provided betweenthe layers of the flexible graphite and further enables contact betweenresin applied on both, i.e. the opposite, sides of the sheet of heatdecomposable carbon-based material in the course of in situ curing ofthe resin. This results in a strong continuous bonding layer of resinbetween, and co-extensive with, the sheet of flexible graphite. Thus, insitu curing of the resin to bond the layers of flexible graphite sheetfor the cover, and decompose the thin sheet of heat decomposable carbonbased material is also preferred.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011]FIG. 1 is a top plan view of a heat shield in accordance with thepresent invention;

[0012]FIG. 1(A) to 1(E) show enlarged portions of FIG. 1;

[0013]FIG. 2 is a side elevation view of the heat shield of FIG. 1;

[0014]FIG. 3 and FIG. 4 show sheets of heat decomposable carbon-basedmaterial for use in the present invention;

[0015]FIG. 5 shows, schematically, the forming of a heat shield inaccordance with the present invention;

[0016]FIG. 6 shows a fragmentary cross-section of a heat shield of thisinvention prior to cooling; and

[0017]FIG. 7 shows a perspective view of a further embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

[0018] Graphite is a crystalline form of carbon comprising atomscovalently bonded in flat layered planes with weaker bonds between theplanes. By treating particles of graphite, such as natural graphiteflake, with an intercalant of, e.g. a solution of sulfuric and nitricacid, the crystal structure of the graphite reacts to form a compound ofgraphite and the intercalant. The treated particles of graphite arehereafter referred to as “particles of intercalated graphite.” Uponexposure to high temperature, the intercalant within the graphitedecomposes and volatilizes, causing the particles of intercalatedgraphite to expand in dimension as much as about 80 or more times itsoriginal volume in an accordion-like fashion in the “c” direction, i.e.in the direction perpendicular to the crystalline planes of thegraphite. The exfoliated graphite particles are vermiform in appearance,and are therefore commonly referred to as worms. The worms may becompressed together into flexible sheets that, unlike the originalgraphite flakes, can be formed and cut into various shapes and providedwith small transverse openings by deforming mechanical impact.

[0019] Graphite starting materials suitable for use in the presentinvention include highly graphitic carbonaceous materials capable ofreversibly intercalating alkali metals. These highly graphiticcarbonaceous materials have a degree of graphitization above about 0.80and most preferably about 1.0. As used in this disclosure, the term“degree of graphitization” refers to the value g according to theformula: $g = \frac{3.45 - {d(002)}}{0.095}$

[0020] where d(002) is the spacing between the graphitic layers of thecarbons in the crystal structure measured in Angstrom units. The spacingd between graphite layers is measured by standard X-ray diffractiontechniques. The positions of diffraction peaks corresponding to the(002), (004) and (006) Miller Indices are measured, and standardleast-squares techniques are employed to derive spacing which minimizesthe total error for all of these peaks. Examples of highly graphiticcarbonaceous anode materials include synthetic graphites and naturalgraphites from various sources, as well as other carbonaceous materialssuch as petroleum cokes heat treated at temperatures above 2500° C.,carbons prepared by chemical vapor deposition or pyrolysis ofhydrocarbons and the like.

[0021] The graphite starting materials used in the present invention maycontain non-carbon components so long as the crystal structure of thestarting materials maintains the required degree of graphitization.Generally, any carbon-containing material, the crystal structure ofwhich possesses the required degree of graphitization, is suitable foruse with the present invention. Such graphite preferably has an ashcontent of less than six weight percent.

[0022] A common method for manufacturing graphite sheet or foil isdescribed by Shane et al. in U.S. Pat. No. 3,404,061, the disclosure ofwhich is incorporated herein by reference. In the typical practice ofthe Shane et al. method, graphite flakes are intercalated by dispersingthe flakes in a solution containing an oxidizing agent such as a mixtureof nitric and sulfuric acid. The intercalation solution containsoxidizing and other intercalating agents known in the art. Examplesinclude those containing oxidizing agents and oxidizing mixtures, suchas solutions containing nitric acid, potassium chlorate, chromic acid,potassium permanganate, potassium chromate, potassium dichromate,perchloric acid, and the like, or mixtures, such as for example,concentrated nitric acid and chlorate, chromic acid and phosphoric acid,sulfuric acid and nitric acid, or mixtures of a strong organic acid,e.g. trifluoroacetic acid, and a strong oxidizing agent soluble in theorganic acid.

[0023] The preferred intercalating agent is a solution of a mixture ofsulfuric acid, or sulfuric acid and phosphoric acid, and an oxidizingagent, i.e., nitric acid, perchloric acid, chromic acid, potassiumpermanganate, hydrogen peroxide, iodic or periodic acids, or the like.Although less preferred, the intercalation solutions may contain metalhalides such as ferric chloride, and ferric chloride mixed with sulfuricacid, or a halide, such as bromine as a solution of bromine and sulfuricacid or bromine in an organic solvent.

[0024] After the flakes are intercalated, any excess solution is drainedfrom the flakes. The quantity of intercalation solution retained on theflakes after draining may range from about 20 to 150 parts of solutionby weight per 100 parts by weight of graphite flakes (pph) and moretypically about 50 to 120 pph. Alternatively, the quantity of theintercalation solution may be limited to between about 10 to 50 parts ofsolution per hundred parts of graphite by weight (pph) which permits thewashing step to be eliminated as taught and described in U.S. Pat. No.4,895,713, the disclosure of which is also incorporated herein byreference. The intercalated graphite flakes are exfoliated by exposingthem to an energy source, such as a heat source like a flame, or energyprovided by infrared, microwave or radiofrequency radiation. In the caseof a flame, the intercalated graphite flakes are advantageously exposedfor only a few seconds, preferably at a temperature greater than about700° C., more typically about 1000° C. or higher.

[0025] The exfoliated graphite particles, or worms, are then compressedand subsequently roll pressed into a densely compressed flexiblegraphite sheet of desired density and thickness and substantiallyincreased anisotropy with respect to thermal conductivity and otherphysical properties. Suitable exfoliation methods and methods forcompressing the exfoliated graphite particles into thin foils aredisclosed in the aforementioned U.S. Pat. No. 3,404,061 to Shane et al.It is conventional to compress the exfoliated worms in stages with theproduct of the first or early stages of compression referred to in theart as “flexible graphite mat.” The flexible graphite mat is thenfurther compressed by roll pressing into a standard density sheet orfoil of preselected thickness. A flexible graphite mat may be thuscompressed by roll pressing into a thin sheet or foil of between about0.05 to 1.75 mm in thickness with a density approaching theoreticaldensity, although a density of about 1.1 g/cm³ is acceptable for mostapplications.

[0026] Roll pressed flexible graphite sheet is known to be a relativelygood thermal barrier in the direction (“c” axis) perpendicular to theparallel planar surfaces of the sheet. The thermal conductivity alongand parallel to the sheet surfaces (“a” axes) is approximately twenty(20) or more times greater than through its thickness (“c” axis).

[0027] With reference to FIG. 1, which is a top plan view, and the sideelevation view of FIG. 2, in a preferred embodiment, a high temperaturereactor is indicated schematically at 10, representing, for example, areactor which involves the use of inorganic halides in a non-oxidizingatmosphere and which operates at temperatures of about 1000° C. andhigher. A heat shielding self-supporting shell is shown at 20. As shownin FIG. 1, the self-supporting shell 20 comprises an anisotropic,continuous spiral wound sheet 25 of graphite. The spiral of flexiblegraphite sheet 25 is suitably from about 1 to 100 mm thick and thedensity of the sheet 25 is suitably from 0.8 to 1.45 g/cm³. Withreference to FIG. 1(A), the transfer of thermal energy through thethickness “t” of the anisotropic flexible graphite sheet 25 (the “c”axis direction) is less than in the plane “l” of the flexible graphitesheet 25 (the “a” axes directions). Thus, most of the heat energyradiated from high temperatures heat source reactor 10 (about 1000° C.and higher) is reflected back to the reactor 10 from the inner surface30 of shell 20, which is formed of anisotropic flexible graphite sheet.Some of the radiant heat energy from reactor 10 is not reflected backand causes the temperature at locations on the inner surface 30 of shell20 to rise. Heat at these locations is rapidly transferred and spread byconduction throughout the anisotropic flexible graphite sheet 25 in alldirections (“1”0 of the “a” axes in the plane of flexible graphite sheet25. Thus, the temperature throughout sheet 25 is essentially uniform andthe presence of persistent hot spots is avoided.

[0028] In order to provide the spiral wound sheet with sufficientstrength to be self-supporting in rugged high temperature environments,a thin layer of cured resin, advantageously an in situ cured phenolicresin, indicated at 33 in FIG. 1, co-extensive with the spiral woundsheet of flexible graphite, is used to bond the spiral wound sheets.Dispersed within this in situ cured resin are typically small particles35 of carbon, shown in FIG. 1(B) resulting from the charring of a heatdecomposable resin supporting substrate during curing of the resin.

[0029] With reference to FIGS. 1(C), 1(D), a thin sheet of heatdecomposable carbon based material or substrate, such as kraft paper asshown at 50 in FIG. 3, or preferably carbon fiber tissue as shown at 50′in FIG. 4, is spiral wound with the anisotropic flexible graphite sheet25 on mandrel 27 as shown in FIG. 5. The thin, heat decomposable carbonbased sheet 50, 50′, co-extensive with flexible graphite sheet 25, isimpregnated with and coated on both sides with liquid resin 60 as shownin FIG. 6. Generally, the resin is coated on each side of sheet 50, 50′to a thickness that can vary between about 5 mm and about 75 mm. Thespiral wound article, before curing of the resin, is shown in FIG. 6.Curing of the resin 60 is accomplished by heating the spiral woundarticle 20 at 125° C. for 16 minutes and 300° C. for 16 hours. In thecourse of curing, the carbon base heat decomposable sheet 50, 50′ isgradually reduced to particles of carbon char (35 in FIGS. 1(B), 1(E))while the gases which evolve from the curing of the resin 60, and thecharring of carbon-based sheet 50, escape from the spiral wound article20 through a temporary channel created by the decomposing of sheet 20and thus do not cause any delamination of the flexible graphite sheet inthe spiral wound article 20. Also, the decomposition of the heatdecomposable sheet into small, isolated particles of carbon enables thecomplete, co-extensive resin bonding of the spiral wound flexiblegraphite sheet as shown in FIG. 1(E).

[0030] The resulting shell is rigid, strong and resistant to corrosion,such as from highly reactive chemical gases, and the cured resin bondingdoes not significantly diminish the thermal properties of the spiralwound shell.

[0031] In a further embodiment of the present invention, illustrated inFIG. 7, a second spiral wound shell 250, identical to the shell 25,except for having a larger cross section, surrounds shell 25, forming anannular chamber 70 therebetween which is at least partially (andpreferably wholly) filled with an insulating material, such asindividual particles 75 of uncompressed expanded graphite. Theseuncompressed particles of expanded graphite receive thermal energy byconduction from the inner shell 25, which is diffused throughout annularchamber 70; any radiant energy from inner shell 25 is likewise diffusedby the particles of expanded graphite 75 and reflected by the inner wall80 of shell 250. The resulting article relatively uniformly reflectsradiant thermal energy back to reactor 10 and maintains an eventemperature profile despite surges in heat radiation from reactor 10while being highly resistant to attack by corrosive gases due to beingformed completely from solid carbon components. The top and bottom ofannular chamber can be sealed by sheets of flexible graphite 82 and 84that can be in the form of the same material as shells 25 and 250, beingprepared in planar form in flat molds, and resin bonded, as indicated at88.

[0032] In the practice of the present invention a suitable resin foruse, such as a phenolic resin like PHYOPHEN 43703 Phenolic Resin inmethanol solvent available from Occidental Chemical Corporation, NorthTonawanda, N.Y. The resin is suitably cured by heating at 125° C. for 16minutes and 300° C. for 16 hours. Kraft paper can be used as the heatdecomposable, carbon based substrate 50. The substrate 50′ can be a PANcarbon fiber tissue or pitch fiber tissue available from Technical FibreProducts Limited, Cumbria, England.

[0033] The above description is intended to enable the person skilled inthe art to practice the invention. It is not intended to detail all ofthe possible variations and modifications that will become apparent tothe skilled worker upon reading the description. It is intended,however, that all such modifications and variations be included withinthe scope of the invention that is defined by the following claims. Theclaims are intended to cover the indicated elements and steps in anyarrangement or sequence that is effective to meet the objectivesintended for the invention, unless the context specifically indicatesthe contrary.

What is claimed is
 1. A thermal insulating device comprising a shellcomprising multiple layers of a continuous spiral wound anisotropicflexible graphite sheet, the layers of spiral wound graphite sheet beingseparated by and bonded to and with a cured resin.
 2. The thermalinsulating device of claim 1 which further comprises a second shellcomprising multiple layers of a continuous spiral wound anisotropicflexible graphite sheet, the layers of spiral wound graphite sheet beingseparated by and bonded to a cured resin, the second shaped shell havinga cross-section larger than that of the shell of claim 1, the secondshell surrounding the shell of claim 1 to define an annular chambertherebetween.
 3. The thermal insulating device of claim 2, wherein theannular chamber comprises a first opening at a first end thereof and asecond opening at a second end thereof, at least one of the first andsecond openings being closed by multiple layers of an anisotropicflexible graphite sheet, the layers of graphite sheet being separated byand bonded to a cured resin.
 4. The thermal insulating device of claim2, wherein the annular chamber has an insulating material containedtherein.
 5. The thermal insulating device of claim 4, wherein theinsulating material comprises particles of exfoliated graphite, carbonfelt, graphite felt, rigid insulation or ceramic wool fibers.
 6. Thethermal insulating device of claim 2, wherein the annular chamber isfilled with a gas.
 7. The thermal insulating device of claim 2, whereina vacuum is drawn in the annular chamber.
 8. The thermal insulatingdevice of claim 1, wherein the shell surrounds a radiant heat source. 9.The thermal insulating device of claim 1, wherein dispersed within theresin are particles of carbon.
 10. A process for preparing a thermalinsulating device, the process comprising forming a shell capable of useas a heat shield surrounding a radiant heat source, by (i) forming ananisotropic flexible graphite sheet (ii) forming the graphite sheet intoa shell comprising multiple layers of a continuous spiral wound graphitesheet, the layers of spiral wound graphite sheet being separated by acurable resin coated on both sides of a thin sheet of heat decomposablecarbon based material co-extensive with said spiral wound sheet offlexible graphite; and (iii) curing the resin, thereby causing at leastpartial decomposition of the carbon based material and enabling contactbetween the resin on each side of the sheet of carbon based material.11. The process of claim 10 wherein the resin comprises a heat-curablephenolic resin.
 12. The process of claim 11 wherein the resin is curedby the application of heat, with decomposition of the carbon basedmaterial creating a channel for escape of gases which evolve from thecuring of the resin.